The need to treat cost effectively water contaminated by hydrocarbons is important in light of increasingly onerous environmental protection regulations.
There are many different methods by which water contaminated by hydrocarbons is currently being treated. These methods are primarily categorized as physical, chemical and biological methods.
Physical methods include air stripping such as tray or packed tower type air strippers: carbon adsorption onto granulated activated charcoal (GAC) units; and, to a lesser extent, ion exchange units and membrane separation (reverse osmosis process).
Chemical methods include precipitation and oxidation/reduction methods.
Biological methods are in wide use today and can be either categorized as aerobic or anaerobic processes. The more common systems are aerobic which are considered to be more effective as they convert the contaminants into innocuous substances. Aerobic systems can generally be designated as suspended growth or fixed film processes.
Suspended growth processes are also referred to as activated sludge type processes where free floating biological flocks are uniformly suspended by aeration in a liquid phase throughout a reactor, thus providing optimal contact of the flocks with the required air and contaminants dissolved in the water.
In a fixed film process, such as a trickling filter, the biomass is attached to a fixed or immobile medium or matrix within the reactor. The waste water is passed over biological slimes grown on and attached to stationary surfaces. These surfaces, also known as trickling filter media, can take various shapes and forms and are often made of plastic or other synthetic materials. Examples of trickling filter media are shown in U.S. Pat. No. 3,403,095, U.S. Pat. No. 3,260,511, Canadian Patent No. 1,061,241 and Canadian Patent No. 1,143,684. The latter patent describes a reactor wherein contaminated water passes downwards through a bed of synthetic packing media, with treated liquid being withdrawn from the bottom of the reactor.
Aerobic biological processes employing partially submersed beds are also used with stationary or, alternatively, moving media such as moving biological contact surfaces, or "contactors". Moving contact media surfaces typically involve rotating surfaces partially submersed in waste water held in a tank and, in many cases, are moving within a rotating cylindrical chamber. Rotating contact media surfaces are intermittently exposed to waste water and to oxygen from the overlying atmosphere, thereby degrading organic constituents in the waste water. Media used are often of metal or plastic construction of various shapes and forms. Examples of such moving contact surfaces are shown in U.S. Pat. No. 4,160,736 "Rotating Trickling Filter", U.S. Pat. No. 4,537,678 "Rotary Biological Contactor" and U.S. Pat. No. 4,549,962 "Rotating Biological Contactor".
Other fixed film bioreactor designs involve wholly submersed or pressurized water systems in which waste water passes downward or upward through contact media surfaces within a bioreactor chamber. Media are often composed of plastic like forms or structures and air is usually bubbled into the base of the bioreactor. An example of such an apparatus is described in a paper entitled "On-site Treatment of Groundwater and Hazardous Waste Using Fixed-Film Bioreactors" presented by Groundwater Technology, Inc. at EnSol 90, Santa Clara convention Center, Santa Clara Calif., Sep. 12-14, 1990. A further example is described in U.S. Pat. No. 4,396,507 "Ribbon Tube Biofilter".
Practices of known prior art teach that, for water contaminated by hydrocarbons, the most useful treatment methods which will satisfy increasingly stringent environmental standards set by regulatory bodies are those which employ naturally occurring microorganisms in submersed or partially submersed reactors to degrade hydrocarbons in an aerobic environment, usually assisted by polishing units, such as granular activated charcoal units, to remove residual traces of organics remaining in the water.
However, treatment processes and the devices used in connection with such processes must strike a balance of economy in construction, operation, maintenance, portability and compliance with minimum emission standards.
There is a need today for a hydrocarbon-contaminated waste water treatment apparatus which best optimizes cost efficiencies in construction, operation, maintenance and portability. Furthermore, such an apparatus must minimize contaminant transfer into the ambient air and be capable of degrading heavier hydrocarbons. Such an apparatus is especially needed for treatment of low water flow rates found in the remediation of polluted properties or effluents from oil-water separator vessels used in the petroleum industry, particularly in operating service stations and bulk plants. In addition, bioreactors that meet hydrocarbon waste effluent treatment standards should have the capability of operating during cold weather without adversely affecting the efficiency of the treatment process.